(a) Field of the Invention
The invention relates to a display system, particularly to an analog front end device of a display system.
(b) Description of the Related Art
Since the 20th century, the development of the television technology and its applications has proved that it is now part of human life and entertainment. Because of the improvement of display technology in recent years, providing massive information and high-definition images has become the guideline of future development of the television industry. Please refer to FIG. 1, showing a schematic diagram of a typical television system and its video signal data source. As shown in FIG. 1, the video signal data source system 110 transmits video data in analog signal format to a television system 120 for displaying a video picture. Although digitized transmission interfaces are already available, analog transmission interfaces remain the most commonly used form of interface.
The video signal data source of the television system 120 can have many forms, such as a DVD player, set top box, and even various game consoles. In general, the components of the video signal data source system 110 comprise a video encoder 112 for performing image data encoding. Then, a digital-to-analog converter (DAC) 114 converts the encoded digital signal into an image analog signal and transmits it.
The television system 120, such as a LCD TV or other flat panel television system or digital television system that is becoming the main stream application, receives an image analog signal transmitted from the video signal data source, converts it into digital format via an analog-to-digital converter (ADC) 124, and performs decoding operation via a video decoder 122 for further image processing and displaying.
There are many types of video encoding formats. The widely seen ones are: RGB format, CVBS format, luminance/chrominance (YC) format, and YPrPb format. Therefore, the analog transmission interface for transmitting video signals between the video source system 110 and the television system 120 can also have several types, such as AV interface for transmitting a signal in CVBS format; S-video interface for transmitting a signal in YC format; and component video interface for transmitting a signal in YPrPb format, etc.
In general, inside an integrated circuit (IC) of the television system 120, a digital circuit and an analog circuit are both included. The digital circuit does not generate offset phenomenon because of the circuit design or influence from manufacturing process, while the analog circuit suffers from the influence of the above-mentioned factors and generates offset or variation, resulting in absolute accuracy distortion phenomenon of the analog-to-digital converter of a single channel or circuit mismatch phenomenon between a plurality of analog-to-digital converters of different channels (for example, between the analog-to-digital converters of the three RGB channels). The factors causing the above-mentioned phenomena comprise the signal gain and offset.
Subsequently, the above-mentioned phenomena will be explained by using the solid line portion of the schematic diagrams illustrated in FIGS. 2 and 3 that show the transfer function of a 2-bit analog-to-digital converter. As shown in FIG. 2, the slope of the dotted line S1 represents a gain value between output and input signal waveform of the analog-to-digital converter. FIG. 2 shows an ideal transfer function of an analog-to-digital converter in which the dotted line S1 passes through the origin of the coordinate axes with slope of 1, representing the offset of the analog-to-digital conversion is equal to zero and the gain of the analog-to-digital conversion is equal to 1. However, the analog-to-digital converter, in reality, cannot achieve the ideal characteristic of an analog-to-digital converter due to imperfect factors, such as process variations. The schematic diagram (solid line portion) illustrated in FIG. 3 shows a practical transfer function of an analog-to-digital converter in which the slope of the dotted line S1 represents the gain value between the output and input signal waveform of an analog-to-digital converter. The dotted line S1 shown in FIG. 3 does not pass the origin of the coordinate axes and the slope is not 1 (it is more than 1, as shown in FIG. 3), representing that the analog-to-digital conversion has offset and the gain is not 1, like the ideal value.
The above-mentioned imperfect characteristic of the analog-to-digital converter causes the deviations of both the transfer function of the analog-to-digital converter and the absolute accuracy from the expectation in the beginning of the design, resulting in the absolute accuracy distortion of the signal of the single channel. In a multi-channel display system, such as an original color image display system that has the R,G, B three channels, the mismatch phenomenon between channels arises due to the difference between channels. For example, in a typical display system with R, G, B three channels that use 8-bit to represent a color value, if the input signal is a gray scale (that is the input value of the three channels are the same), due to the above-mentioned channel mismatch resulting from the imperfect characteristic of the analog-to-digital converter, the digital value generated by each channel may not be the same and thus the color shift phenomenon may be generated.
Although the offset phenomenon generated by the analog-to-digital conversion can be calibrated by the black level transmitted during the porch interval, there is no effective solution for the imperfect characteristic of the gain, as mentioned in the above.